The present invention generally relates to methods and equipment for monitoring ECG waveforms and more particularly to methods and equipment for ambulatory ECG monitoring, display and analysis.
Ambulatory ECG monitoring is a standard medical diagnostic technique whereby patient electrocardiogram data can be monitored over an extended period, for example 24 hours, by tape recording ECG waveforms using a portable unit operated by the patient, which tape recording is subsequently played back and analyzed in a laboratory, physician's office or other convenient facility.
Normally, the data is originally recorded on the tape at a speed of 33/4 inches per minute and then replayed during analysis at an accelerated rate of 33/4 or 71/2 inches per second. Consequently, the ECG data is displayed to the reviewing technician on an oscilloscopic display at a rate of 60 or 120 times real times speed. The goal of the technician is to find and hard copy abnormal segments of cardiac rhythm for subsequent physician review.
In one prior art display technique, which is disclosed in U.S. Pat. No. 3,215,136 issued to Holter, et al, waveforms representing heart beats; commonly referred to as QRS complexes, are superimposed over each other. The reviewing technician is then expected to discriminate abnormal complexes by their lack of superposition. One problem which was initially encountered in this technique was due to the fact that QRS complexes are asynchronous. Consequently, when displaying these repetitive, asynchronous QRS complexes on a self triggering oscilloscope, the trace patterns would vary along the horizontal or x axis so widely that they would not overlap and a reliable superimposition comparison was difficult, if not impossible.
As disclosed in U.S. Pat. No. 3,229,687 issued to Holter, et al, an attempt to solve this problem of lack of superimposition entailed the use of two pick-up heads which are spaced apart, thereby causing the signal picked up by the second head to be delayed by a predetermined amount of time. The first pick-up head would send a signal to the oscilloscope causing the sweep to start upon the detection of the R wave. The delayed signal from the second head would then be applied to the input of the oscilloscope thereby causing the entire complex to be displayed. Although the subsequent technique improved the degree of superimposition, this technique of displaying abnormalities has some fundamental problems associated therewith. For example, the superimposition technique presents the abnormality to the receiving technician for only a fraction of a second, devoid of its rhythm context. Therefore, a detailed examination of a given ECG sequence requires the stopping of the rapid scan and printing out of the ECG rhythm in real time. In addition, requirements on the operator of total concentration on a superimposed display of 100 to 200 complexes per second as well as the pressure which exists to complete the scan of a 24 hour recording in a reasonable amount of time while constantly stopping the scan to examine and verify questionable segments, can lead to operator fatigue and inaccuracies in data identification.
Some superimposition scanners rely heavily upon analog and digital computer arrhythmia detectors to electronically count and categorize abnormal beats. These detectors often miss abnormal beats, commonly referred to as false negatives, or often count electronic noise or movement artifacts, commonly referred to as false positives, thereby yielding erroneous results. These types of systems are so designed that reliable human review of the correctness of the computer counts and categorizations of abnormal beats is difficult to accomplish.
Another technique, as disclosed in U.S. Pat. No. 3,853,119, issued to Peterson, et al, involves the use of a continuous rhythm scanner which presents a predetermined time segment, for example 2 minutes, of digitally sampled ECG data on a large screen monitor in stationary display to the reviewing technician, at a rate controlled by the technician. Although this technique represents an improvement over the superimposition technique, the ECG data presented to the reviewing technician is merely an approximation of the original analog ECG signal and may be of poor quality due to the relatively slow sampling rate employed in the digitization. In order to improve the quality of the displayed ECG signals to the American Heart Association recommended standard of 0.1 to 100 Hz, a digital continuous rhythm scanner, operating at taped playback speeds of 120 times real time speed, would have to sample at a rate of at least 24,000 samples per second. Although these high sampling rates are achievable using equipment which is presently available, this equipment is relatively expensive, making this technique uncompetitive with the less expensive superimposition method.